1. Field of the Invention
The present invention relates to an antenna device, and more specifically, to an antenna device used for a base station in a radio communication system with a submillimeter wave band or a millimeter wave band.
2. Description of the Related Art
Installation of optical fibers is being tried in order to achieve effective and high-speed transmission of a large amount of data in the current information-oriented society; however, high installment cost results in boosting the ratio of the total communication fees of ordinary households to access line fees (fixed fees portion).
Under these circumstances, establishing a radio communication system with a millimeter wave band (local radio network WLL) is now drawing attentions. This system makes it unnecessary to install lines up to users"" houses and further provides a local area with services unique to the area at a low cost.
The important questions in establishing a radio communication system are how many terminal users each base station can accommodate and how fast the system can transmit large amounts of information accurately. As one solution of the questions, developing an antenna device is an urgent necessity.
As a base station antenna in the radio communication system with the millimeter wave band, using an antenna device with a horn antenna has been considered. The antenna device must be provided with wide radiation characteristics in a horizontal plane in order to cover a large service area.
FIG. 19 show s a prior art antenna device employing a horn antenna. The horn antenna 102 has the geometry of a pyramid in which the width A1 of an antenna opening 102a is larger than the width of the opening portion 101a of a waveguide 101.
However, the horn antenna often employed in a radio communication with a millimeter wave band is generally used as a high-gain antenna with a narrowed beam, so that the half-power beamwidth is generally narrow.
In view of the aforementioned problem, the main object of the present t invention is to secure wide radiation characteristics In a horizontal plane, in spite of the antenna device having a simple structure.
In order to achieve the object, the antenna device of the present invention comprises a plurality of directional antennas which are aligned on the same axis in one direction and which are also arranged in a manner that the main radiation directions of the plurality of directional antennas deviate from each other in a direction orthogonal to the direction. In this structure the radio waves from the directional antennas are radiated at least with the point coinciding with the same axis in the one direction as its center, so that the occurrence of a drop in directivity due to the array factor is prevented. Since the directivity in the antenna device is the combination of the directivities of all the directional antennas, the main radiation directions of the directional antennas can be deviated from each other in the one direction to expand the radiation characteristics in the plane orthogonal to the one direction.
The above-mentioned one direction is preferably a vertical direction, which could expand the radiation characteristics in the horizontal plane.
The antenna device preferably comprises a distribution circuit for supplying the plurality of directional antennas with electric powers.
In this case, various directivities can be obtained according to the following variations.
The distribution circuit supplies the plurality of directional antennas with electric powers of the same amplitude and the same phase.
The distribution circuit supplies the plurality of directional antennas with electric powers of the same amplitude and different phases.
The distribution circuit supplies the plurality of directional antennas with electric powers of the same phase and different amplitudes.
The distribution circuit supplies the plurality of directional antennas with electric powers of different amplitudes and different phases.
It is preferred that the antenna device further comprises a power supply part waveguide, and that the plurality of directional antennas are horn antennas connected to the power supply part waveguide. In this case, the half-power beamwidth can be further increased by making at least one of the plurality of directional antennas have an antenna opening shorter in width than the opening portion of the power supply part waveguide.
It is preferred that the antenna device further comprises a fixing plate which is arranged in a direction orthogonal to the one direction, wherein the plurality of directional antennas are provided with respective placing surfaces which are arranged parallel to the direction orthogonal to the one direction and which are in contact with the front and back sides of the fixing plate so as to fix the antenna device thereonto. In this case, the angles of the directional antennas made of horn antennas can be freely set by providing an installment angle setting unit for freely adjusting the main radiation directions of the plurality of directional antennas fixed on the fixing plate.
In an antenna device comprising a horn antenna connected to a power supply part waveguide, the half-power beamwidth can be increased by making the antenna opening of the horn antenna shorter in width than the opening portion of the power supply part waveguide.
In this case, the side surfaces of the horn antenna are tapered from the opening portion of the power supply part waveguide to the antenna opening. This makes the antenna opening of the horn antenna shorter in width than the opening portion of the power supply part waveguide, without causing a large drop in gain or increasing the internal volume of the antenna.